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The difference between stack and heap memory is common programming question asked by beginners learning Java or any other programming language. Stack and heap memory are two terms programmers starts hearing once they started programming but without any clear and definite explanation. Lack of knowledge of what is a heap in Java and what is stack memory in Java results in misconceptions related to stack and heap. To add to this confusion, a stack is also a data structure which is used to store elements in LIFO(Last In First out) order and available in Java API as java.util.Stack. In general, both stack and heap are part of memory, a program is allocated and used for different purposes. Java program runs on JVM which is launched as a process by "java" command. Java also uses both stack and heap memory for different needs. In our last article 10 points on Java heap space, I have touched base on Java heap space and in this article we will see the difference between stack and heap memory in Java.

Difference between Stack vs Heap in Java

Here are few differences between stack and heap memory in Java:

1) The main difference between heap and stack is that stack memory is used to store local variables and function call while heap memory is used to store objects in Java. No matter, where the object is created in code e.g. as a member variable, local variable or class variable, they are always created inside heap space in Java.

2) Each Thread in Java has their own stack which can be specified using -Xss JVM parameter, similarly, you can also specify heap size of Java program using JVM option -Xms and -Xmx where -Xms is starting size of the heap and -Xmx is a maximum size of java heap. to learn more about JVM options see my post 10 JVM option Java programmer should know.

3) If there is no memory left in the stack for storing function call or local variable, JVM will throw java.lang.StackOverFlowError, while if there is no more heap space for creating an object, JVM will throw java.lang.OutOfMemoryError: Java Heap Space. Read more about how to deal with java.lang.OutOfMemoryErrorin my post 2 ways to solve OutOfMemoryError in Java.

4) If you are using Recursion, on which method calls itself, You can quickly fill up stack memory. Another difference between stack and heap is that size of stack memory is a lot lesser than the size ofheap memory in Java.

5) Variables stored in stacks are only visible to the owner Thread while objects created in the heap are visible to all thread. In other words, stack memory is kind of private memory of Java Threads while heap memory is shared among all threads.

That's all on the difference between Stack and Heap memory in Java. As I said, It’s important to understand what is heap and what is stack in Java and which kind of variables goes where, how you can run out of stack and heap memory in Java etc. Let us know if you are familiar with any other difference between stack and heap memory in java.

The stack contains only values for integral types and references to objects, which are always stored in the heap. Since it doesn't contain any objects itself, there would be nothing to garbage-collect.

"No matter, where object is created in code e.g. as member variable, local variable or class variable, they are always created inside heap space in Java."

This is inaccurate. Escape analysis was introduced in 6u14 and enabled by default in 6u23.

"The -XX:+DoEscapeAnalysis option directs HotSpot to look for objects that are created and referenced by a single thread within the scope of a method compilation. Allocation is omitted for such non-escaping objects, and their fields are treated as local variables, often residing in machine registers. Synchronization on non-escaping objects is also elided."

http://www.oracle.com/technetwork/java/javase/6u14-137039.html

"Escape analysis is supported and enabled by default in Java SE 6u23 and later."

Heap memory is used by all the parts of the application whereas stack memory is used only by one thread of execution.Whenever an object is created, it’s always stored in the Heap space and stack memory contains the reference to it. Stack memory only contains local primitive variables and reference variables to objects in heap space.Objects stored in the heap are globally accessible whereas stack memory can’t be accessed by other threads.Memory management in stack is done in LIFO manner whereas it’s more complex in Heap memory because it’s used globally. Heap memory is divided into Young-Generation, Old-Generation etc, more details at Java Garbage Collection.Stack memory is short-lived whereas heap memory lives from the start till the end of application execution.We can use -Xms and -Xmx JVM option to define the startup size and maximum size of heap memory. We can use -Xss to define the stack memory size.When stack memory is full, Java runtime throws java.lang.StackOverFlowError whereas if heap memory is full, it throws java.lang.OutOfMemoryError: Java Heap Space error.Stack memory size is very less when compared to Heap memory. Because of simplicity in memory allocation (LIFO), stack memory is very fast when compared to heap memory.